Method and apparatus for identifying the components of a signal

Abstract

Fast Fourier Transform methods of analyzing signal frequencies spectra yield results from zero frequencies upwards. Thus, when the frequency band of interest is not at very low frequencies a lot of wasted calculations may be performed, and resolution and computation time have to be traded-off against each other, often falling short of the requirements for both. A technique is described in which the signal to be analyzed is passed through an anti-alias filter and initial transform data is collected by undersampling. In a railway track circuit receiver which is to identify a predetermined carrier frequency or FSK signal, the anti-alias filter is selected to exclude frequencies other than those in a frequency band including the particular track signal so that there is no ambiguity in the calculated transform results.

I claim: 
     
       1. A method of analysing a signal, for the purpose of identifying its frequency components, by performing a fast Fourier transform on real time series samples of the signal, including, in combination, the steps of filtering to exclude alias frequencies of the signal and sampling the filtered signal at a rate substantially lower than the minimum rate required to unambiguously identify the frequency of the signal. 
     
     
       2. A method according claim l, wherein the digital sampling rate is no greater than approxmately half the frequency of the lowest expected frequency component of the signal. 
     
     
       3. A method as claimed in claim 1, further including the step of applying a window function to signal samples. 
     
     
       4. Apparatus for performing the method of claim 1, including means for evaluating a fast Fourier transform algorithm, and data collection means for obtaining real time series samples of a signal, which comprises an anti-alias filter having an output connected with digital sampling means operative to sample the filtered signal at a rate substantially lower than the minimum rate required to unambiguously identify the frequency of the signal. 
     
     
       5. Apparatus as claimed in claim 4, wherein the anti-alias filter is a band-pass filter having a bandwidth equal to half the sampling frequency. 
     
     
       6. Apparatus as claimed in claim 5 adapted for identifying a modulated carrier signal, wherein the anti-alias filter has a centre frequency at substantially the same frequency as that of the carrier signal. 
     
     
       7. Apparatus as claimed in claim 4, wherein the data collection means includes means for applying a Hanning window function to the data samples collected during one cycle of the apparatus. 
     
     
       8. Apparatus as claimed in claim 7, wherein the Hanning window means comprises means for multiplying a data sample by factor determined in accordance with a normalised cosine squared periodic function having a half-period equal to one sampling cycle. 
     
     
       9. Apparatus as claimed in claim 1, wherein there is provided means responsive to the transformation results for performing a ratio test to determine a frequency.